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package org.apache.juneau.http;

/**
 * Represents a parsed <l>If-Unmodified-Since</l> HTTP request header.
 * 
 * <p>
 * Only send the response if the entity has not been modified since a specific time.
 * 
 * <h5 class='figure'>Example</h5>
 * <p class='bcode'>
 * 	If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
 * </p>
 * 
 * <h5 class='topic'>RFC2616 Specification</h5>
 * 
 * The If-Unmodified-Since request-header field is used with a method to make it conditional.
 * If the requested resource has not been modified since the time specified in this field, the server SHOULD perform the
 * requested operation as if the If-Unmodified-Since header were not present.
 * 
 * <p>
 * If the requested variant has been modified since the specified time, the server MUST NOT perform the requested
 * operation, and MUST return a 412 (Precondition Failed).
 * 
 * <p class='bcode'>
 * 	If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
 * </p>
 * 
 * <p>
 * An example of the field is:
 * <p class='bcode'>
 * 	If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
 * </p>
 * 
 * <p>
 * If the request normally (i.e., without the If-Unmodified-Since header) would result in anything other than a 2xx or
 * 412 status, the If-Unmodified-Since header SHOULD be ignored.
 * 
 * <p>
 * If the specified date is invalid, the header is ignored.
 * 
 * <p>
 * The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an
 * If-Modified-Since header fields is undefined by this specification.
 * 
 * <h5 class='section'>See Also:</h5>
 * <ul class='doctree'>
 * 	<li class='extlink'><a class='doclink' href='https://www.w3.org/Protocols/rfc2616/rfc2616.html'>Hypertext Transfer Protocol -- HTTP/1.1</a>
 * </ul>
 */
public final class IfUnmodifiedSince extends HeaderDate {

	/**
	 * Returns a parsed <code>If-Unmodified-Since</code> header.
	 * 
	 * @param value The <code>If-Unmodified-Since</code> header string.
	 * @return The parsed <code>If-Unmodified-Since</code> header, or <jk>null</jk> if the string was null.
	 */
	public static IfUnmodifiedSince forString(String value) {
		if (value == null)
			return null;
		return new IfUnmodifiedSince(value);
	}

	private IfUnmodifiedSince(String value) {
		super(value);
	}
}
